Conventionally, there have been proposed various types of vehicle travel control devices which control vehicle trajectory so that a vehicle travels along a target trajectory. For example, Japanese Patent Application Laid-Open (kokai) No. 2001-48035 discloses a lane keeping device which sets a target trajectory on the basis of lane information of a road in front of a vehicle and controls an automatic steering actuator so that a difference between a target trajectory and a point of regard of a driver in front of a vehicle.
According to the lane keeping device described in the reference, it is possible to promote a driver to make a steering operation or to modify steered angle of the steerable wheels so that a vehicle travels along a target trajectory by means of control of steering torque.
However, in the lane keeping device described in the reference, a vehicle exterior information acquisition means such as a camera is essential which acquires lane information of a road in front of a vehicle for setting a target trajectory and a point of regard of a driver in front of a vehicle corresponding to an actual trajectory. Since a target trajectory is fundamentally determined on the basis of lane information, a target trajectory can not always be set to a trajectory which a driver expects. In addition, under a situation where no lane information exists in front of a vehicle or no lane information can be acquired, a target trajectory can not be set and accordingly, a vehicle can not be driven along a target trajectory.
It is to be noted that Japanese Patent Application Laid-Open (kokai) No. 2007-269180 discloses a steering control device which sets a target trajectory on the basis of steering angle and vehicle speed and controls a steering transfer ratio so that a difference between a target trajectory and an actual trajectory of a vehicle decreases. According to the steering control device described in the reference, steered angle of the front wheels can be controlled so that a vehicle travels along a target trajectory.
However, in the steering control device described in the reference, a vehicle exterior information acquisition means such as a GPS for determining an actual trajectory is required. Since steered angle of the front wheels is controlled in a feed-back manner on the basis of a difference between a target trajectory and an actual trajectory, steered angle can not be controlled unless an actual trajectory has been determined and accordingly, a vehicle can not be controlled desirably along a target trajectory due to control delay.